But if you're intent on using HTML/CSS the closest you can come (within the standard) is setting a background image of a bracket that stretches the whole height of your containing element. This means that whatever image you use will probably have a narrow range of heights where it looks okay before it's too distorted to look good.
If only CSS had an element height unit like: https://tomhodgins.github.io/element-queries-spec/element-queries.html#eh
… then we would be able to create brackets that had the same `font-size` as the height of their container.
Here's my solution for scalable surrounding brackets in EQCSS: http://codepen.io/tomhodgins/pen/xgPZew?editors=1100